(a) Field of the invention
The present invention relates to a photographic lens system, for use in a compact camera having a range finder and using a 35 mm film, and more particularly to a lens system having an angular field of view of 60.degree. to 70.degree. and an aperture ratio of F/2.8, the length from the first lens surface to the film surface being almost as short as the overall focal length.
(b) Description of the prior art
Conventionally, for the purpose of making compact a lens such as described above, a Tessar type lens system with a rear aperture stop has been adopted. However, in a lens system of Tessar type with a rear aperture stop (as described, for example, in Japanese Published Examined Patent Application No. 19387/72), the length from the first lens surface to the film surface had to be limited to a length in the order of 42 mm for use in a 35 mm film.
In order to eliminate this disadvantage, the inventor of the present application has designed a very compact lens system and has filed it as a Japanese patent application (See Japanese Published Unexamined Patent Application No. 76147/79). In this lens system, the length from the first lens surface to the film surface is made so short as to be about the same as the focal length.
However, in either of the lens system disclosed in the above mentioned Japanese Published Examined Patent Application No. 19387/72 and Japanese Published Uniexamined Patent Application No. 76147/79, there have been defects that, as the stop is not positioned in the middle of the lens system, it is difficult to well correct various aberrations and, when a small stop aperture is selected, no sufficient light amount of the marginal part of the image field will be obtained.
The present invention is to provide a very compact photographic lens system wherein the length from the first lens surface to the film surface is not more than 35 mm and various aberations are well corrected not only in the central part but also in the marginal part of the image plane.
In the photographic lens system disclosed in the above mentioned Japanese Published Unexamined Patent Application No. 76147/79, even if the stop located between the third lens component and the fourth lens component is intended to be moved toward the object side in order to be positioned substantially in the middle of the lens system, there will not be space in which the stop can be arranged. Therefore, in this known lens system, it is impossible to substantially symmetrically arrange the respective lens components, hence the above described defects. That is to say, in this known lens system, the same as in the lens system disclosed in Japanese Publish Examined Patent Application No. 19387/72, the pencil of rays is asymmetrical with respect to the center of the stop and, as a result, it is hard to correct the aberrations in the formation. Further, it is difficult to well balance the corrections of the aberrations when the stop aperture is fully opened and when it is made small and no sufficient light amount of the marginal part of the image field can be obtained.
In the lens system according to the present invention, in order to position the stop toward the object side as much as possible, the third lens component which is a positive lens is separated into two lens elements. There is provided a space in which the stop can be arranged between these two separated lens elements, hence the lens system of such formation as is shown in FIG. 2. The power distribution to the respective lens components is the same as in the photographic lens system disclosed in Japanese Published Unexamined Patent Application No. 76147/79. That is to say, the power distribution is so made as to satisfy the later described conditions (1) and (2) and the features (of being of a wide angle and yet very compact) of the above mentioned known photographic lens system are perfectly provided.
The means for obtaining a lens system provided with the above mentioned features shall be explained in the following.
In order to shorten the total length of the lens system, there have been known two methods, one is to shorten the focal length of the lens system and the other is to make the telephoto ratio small (a proportion of the length from the first lens surface to the film surface, to the focal length). In order to make the total length of the lens system markedly short as described above, it is necessary both to shorten the focal length and to make the telephoto ratio small. In order to make the telephoto ratio small, there may be adopted a telephoto type lens system in which a negative lens is arranged in the rear portion of the lens system. However, in order to shorten the focal length in such a telephoto type lens system, it is necessary to secure good lens characteristics over an angular field of view wider than 60.degree. which was inconceivable in a conventional telephoto type lens system.